This invention relates to the selective recovery of copper from various acidic solutions which may also contain nickel, cobalt, iron, and other ions. More specifically, it relates to a continuous process for recovering copper from acidic leaching liquors containing cupric ions in admixture with other metal ions and to a process useful as a recycle step for the process disclosed in U.S. Application Ser. No. 695,389 by A. S. Rappas, et al, filed on even date herewith, entitled Separation and Recovery of Copper Metal from Ammoniacal Solutions, the teachings of which are incorporated herein by reference.
In it known that copper metal can be precipitated from solutions containing complexed cuprous ions by disproportionation of the complex according to the following reactions: EQU 2CuL.sub.2.sup.+ .revreaction. 2Cu.sup.+ + 4L EQU 2cu.sup.+ .revreaction. Cu.degree. + Cu.sup.+.sup.+
where L is a cuprous ion stabilizing ligand. U.S. Pat. No. 3,865,744 by A. J. Parker et al., broadly describes various methods of production and disproportionation of cuprous-nitrile complexes in acidic solutions. Among the methods included therein for the reduction of cupric ions to cuprous nitrile complexes are: reduction with SO.sub.2 gas to Chevreul's salts and dissolution of the latter in the presence of nitriles to give the cuprousnitrile complexes; dissolution of crude copper metal with cupric solutions containing nitrile; and reduction of cupric to cuprous nitrile complexes by various metallic reducing agents, e.g., Ag, Fe, Ni, Zn, Cd, Co, Sn, etc.
Among the methods which achieve reduction of cupric ions to metallic copper (powders), it appears that the most attractive for industrial applications would be the SO.sub.2 reduction process. However, the use of SO.sub.2 as a reducing agent requires relatively high temperature and pressure, is very pH sensitive, is relatively slow, and is characterized by copper yields between 50 and 75% and low copper product purity necessitating further processing.
U.S. Pat. No. 870,786, to Jamau, broadly describes the reduction of cupric sulfate solutions to copper metal, whereas U.S. Pat. No. 1,201,899 to Weidlein teaches carrying out this reduction at specific temperatures and pressures, e.g., at 150.degree. C. and 100 psi. However, experience shows that recovery of copper by these methods rarely exceeds 50% by weight of the copper in solution. U.S. Pat. No. 3,880,647, to N. R. Tipman, discloses the use of certain catalysts such as dimethyl sulphoxide or dimethyl formamide for increasing the recovery of copper from solutions when the reduction is done with SO.sub.2.
It would be very advantageous if such a reduction of cupric ions in solution could be performed with hydrogen gas since this reducing agent is relatively inexpensive, clean and results in acidified water as an effluent. EQU Cu SO.sub.4 + H.sub.2 .sup.H .sbsp.2O Cu.degree. + H.sub.2 SO.sub.4
thermodynamically, this reduction is possible. However, kinetically, it requires high pressures and temperatures as well as strict pH control, and thus the use of expensive equipment; and, the copper product is impure and requires further processing. U.S. Pat. Nos. 3,833,351 and 3,877,931, to D. R. Neskora et al. and D. R. Neskora, respectively, describe such a process for reduction of copper from solution using reducing gases, e.g., hydrogen, in continuous flow tubular reactors.
The instant invention arose from the realization that it would be highly advantageous if cupric solutions could be reduced with hydrogen gas in the presence of acetonitrile to produce a cuprous-nitrile complex which could subsequently be disproportionated in a separate reactor to yield very high purity copper metal. Manifestly, direct hydrogen reduction to produce cuprous-nitrile complexes under the conditions recited above is not feasible since, under the same conditions of temperature and pressure required, the nitriles are completely destroyed by hydrolysis according to the equation: ##STR1## where R is an alkyl group, e.g., --CH.sub.3.